The present invention relates to a multi-indicia fluorescent display tube. More particularly, the present invention relates to such a display tube of the diode type suitable for use in various small-sized electronic apparatus such as electronic timepieces of the digital display type, which is actuated by a driving circuit of the static drive type (hereinafter referred to as a static driving circuit) and is not provided with a control electrode or grid for controlling anode current and, in addition, can display a plurality of patterns such as characters, figures and symbols in a row by emission of uniform fluorescent light.
Generally, a fluorescent display tube uses fluorescent material able to luminesce by excitement due to bombardment of electrons thereon from the cathode, and thereby can obtain very clear luminescent display. Therefore, it is widely used for digital read-out display devices of various electronic equipment such as small-sized desk calculators and digital electronic clocks. Recently, a new fluorescent tube of the so-called multi-indicia type has come into general use, which has a plurality of pattern display sections provided side by side in a row in an air-tight vacuum casing for displaying patterns such as characters. Such a display tube generally comprises a plurality of pattern display sections provided in a row and each consisting of a group of segment anodes each having a fluorescent material layer thereon, a filament-shaped direct-heated cathode (hereinafter referred to as a filament cathode) for emitting electrons toward the pattern display sections, and control electrodes provided between each pattern display section and the cathode for accelerating and controlling electrons emitted from the cathode; and is usually actuated by a driving circuit of the dynamic drive type. However, when a fluorescent display tube is operated by such a driving circuit of the dynamic drive type, a pulse current flow, which may cause various types of noises, tends to appear in the display tube and the driving circuit. Therefore, such a display tube cannot be used for a display readout device of an electronic timepiece or the like incorporated in an electronic equipment which must be insulated from acoustical or video noises.
Therefore, in the case of such an electronic equipment which must be insulated from the above-mentioned noises, it is desirable to use a display tube which can be operated by the so-called static driving circuit without using a pulse power source. Provision of a static driving circuit makes it possible to use a multi-indicia display tube of the diode type, because direct current flows between the cathode and the segment anodes of the pattern display sections or indicia selected according to patterns such as characters to be luminescently displayed and therefore a control electrode between the cathode and each group of anodes can be eliminated. In the case of such a display tube of the diode type having no control electrode, the portions of and above the surface of the base plate around and in the vicinity of each segment anode tend to be non-uniformly and negatively charged with electrons emitted from the cathode, though a transparent conductive film electrically connected to the cathode is provided on the portion of the inside surface of the casing above the display sections so as to shield the tube from external electrical fields. If such electrification once occurs, the normal electric field and electron current between the cathode and each anode will be disturbed resulting in uneven or defective or, what is worse, insufficient luminescence of the fluorescent material layer on the segment anode.
For this reason, a fluorescent display tube used with a static driving circuit is heretofore also provided with a mesh-shaped auxiliary control electrode between the cathode and a plurality of pattern display sections. Thus, by applying a positive potential to the auxiliary control electrode with respect to the cathode potential, the display tube can obtain the normal electric field and electron current not influenced by the above-mentioned electrification between the cathode and anode.
However, the above-mentioned auxiliary control electrode must be positioned at a very close distance (usually about 0.3 mm or less) from the pattern display section composed of a group of segment anodes. In addition, it must be formed of a net of very fine meshes produced from a very thin metal sheet (about 0.05 mm thick) so as to clearly see the luminescent display of the pattern display section from above. Therefore, such a close distance of about 0.3 mm or less between the mesh-shaped auxiliary electrode and pattern display section will cause the electrification of the portions around and in the vicinity of each segment anode on the base plate to have an influence which results in insufficient luminescent display. Moreover, in the case of a fluorescent display tube having pattern display sections adapted to show larger-sized patterns, the auxiliary control electrode, which is formed of a mesh-shaped thin metal sheet, must be made large in size and therefore becomes liable to deformation such as expansion or contraction due to the change of ambient temperature or the like during or after the production process of the tube with the result that the control electrode and adjacent segment anode cannot be held at a proper distance or, what is worse, comes into contact with each other. For this reason, it has been very difficult to make the fluorescent display tube with an auxiliary control electrode of the above-mentioned construction suitable for use in a digital timepiece or the like adapted to show larger-sized patterns.
Besides, there have been also proposed various kinds of fluorescent display tubes of the static drive type in which a control electrode is provided around and in the vicinity of each pattern display section composed of a group of segment anodes and, at the same time, substantially flush with the segment anodes, without providing the above-mentioned mesh-shaped auxiliary control electrode. The use of such a control electrode known as a flat-type control electrode makes it possible to produce the tube very easily and inexpensively, to eliminate the possibility of the control electrode being deformed as in the case of the mesh-shaped control electrode, and to eliminate the need for the above mesh-shaped shield.
However, in the above-mentioned case, there is provided an insulating layer between the segment anodes on the base plate and the flat control electrode surrounding the segment anodes. Therefore, the surface of the insulating layer tends to be electrified during operation and thereby the electrical field between the cathode and anode is disturbed resulting in uneven luminescence of the pattern display sections.
The above-mentioned fluorescent display tube having flat-type control electrodes has a potential distribution between the cathode and anode during operation as shown in FIG. 1. Electron currents 4 flow from filaments of the cathode 1 to segment anodes 3 on the base plate 2 as shown, under the influence of an electric field produced by potentials applied to the cathode 1, a transparent conductive layer provided on the inside surface of the window portion of the cover plate and electrically connected to the cathode so as to shield the tube from external electric fields, the segment anodes 3, and flat control electrodes 5 provided in the vicinity of the respective groups of the segment anodes 3 and on a plane substantially flush therewith. Since the control electrode 5 is substantially on the same plane as the segment anode 3, the electric field produced is not smooth and therefore its electronic optical effect makes it impossible to obtain a uniform density of the electron current 4 flowing from the cathode and the segment anode with the result that the fluorescent material layer 6 on the segment anode 3 luminesces unevenly. Especially, in the case of a digital electronic clock or the like which displays patterns larger in size, the above-mentioned uneven luminescence poses serious problems. In the foregoing case, the density of the electron current can be levelled to some extent by increasing the number of cathode filaments, but the increase in the number of cathode filaments will result in an increase in power consumption.